mittepositiivsetel
Mittepositiivsetel appears to be a nonstandard or informal inflection related to the Finnish adjective mittepositiivinen, meaning non-positive. In standard Finnish, the adjective declines through cases and numbers in regular patterns, and the commonly used forms for plural nouns in the adessive would be kuten mittepositiivisillä arvoilla (on non-positive values), rather than mitterepositiivsetel. As a result, mitteipositiivsetel is not recognized as a standard grammatical form in Finnish grammar, and it is rarely if ever found in formal mathematical or linguistic writing.
The linguistic note is that Finnish adjectives typically align with the nouns they modify in case and
In mathematical contexts, the concept closest to the potential meaning of mittepositiivsetel would be non-positive numbers—numbers
See also: mittepositiivinen, mittepositiiviset, non-positive numbers, mathematical terminology in Finnish.